1. Field of the Invention
The present invention relates to a torque sensor of the noncontact type and, more particularly, to a torque sensor for converting the torque of a rotational shaft into an electric signal.
2. Discussion of Background
It has been recently required in a system for generating a rotary force, such as an engine system, to control a system for generating the rotary force in response to a signal from a sensor which senses the rotary torque. Thus, a variety of sensors for sensing the rotary torque have been proposed. For examples, see the article by William J. Fleming and Paul W. Wood entitled, "Noncontact Miniature Torque Sensor for Automotive Application," SAE paper 830206, presented at the Automotive Engineering Congress; the article by K. Harada, I. Sasadas, T. Kawajiri, and M. Inoue entitled, "A New Torque Transducer Using Stress Sensitive Amorphous Ribbons," IEEE Transactions on Magnetics, Vol. MAG-18, No. 6, November 1982; and also see Japanese Patent disclosure (Kokai) No. 57-211030 (corresponding to U.S. application Ser. No. 268,890 filed on June 1, 1981). In these torque sensors, the following problems have been pointed out.
In the torque sensor proposed by Fleming et al in "Noncontact Miniature Torque Sensor for Automotive Application," the torque is measured by the variation in the magnetic properties of the engine crankshaft in response to the torque applied to the shaft, i.e., the magnetic permeability. However, there is also the problem such that the magnetic properties of the measuring range of the engine crankshaft are not uniform making it difficult to accurately measure the torque due to the magnetic anisotropy on the surface and in the crankshaft. Since the engine crankshaft itself does not have large magnetic permeability, it is necessary to generate a magnetic flux sufficient to penetrate the measuring range of the crankshaft and to be detected, with the adverse result that the device for generating the magnetic flux must be large in size. In contrast, K. Harada et al in "A New Torque Transducer Using Stress Sensitive Amorphous Ribbons" and Japanese Patent Disclosure (KOKAI) No. 57-211,030 describes a torque sensor in which an amorphous magnetic ribbon is bonded to a shaft, and in which a coil assembly is provided coaxially with the axis of the shaft. The torque sensor generates a magnetic flux parallel with its axis which measures the permeability of the amorphous magnetic ribbon which varies in response to the torque applied to the shaft. This torque sensor prevents the possibility of an erroneous detection based on the irregular magnetic properties of the material. However, a relatively large reverse magnetic field is produced when the coil assembly generates a magnetic flux in parallel with the axis of a shaft. Accordingly, there is the problem that a relatively large exciting current may be supplied to the coil assembly. Since the coil assembly is provided around the shaft, it is necessary to form a space for it, and so the sensor cannot be readily assembled into the system for generating rotary force, and the space for associating the sensor cannot be obtained in some systems.